I have two similar iMacs (27" - i7 - os10.8.4 ) and use Apple's extended USB keyboards on both of them. After a power outage neither keyboard will work on one iMac (A) although both keyboards work fine on the other iMac (B). I have reset NVRAM and SMC on iMac A but to no avail. I have plugged printers into the USB ports and they seem to work fine. As it stands, the only way I can log in on iMac A is by using a Bluetooth keyboard, the USB port however remains "invisible"...

I presume your question is how can you reconnect the keyboards to the USB port on iMac A?
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DaveAug 26 '13 at 21:43

Yes Dave, that is my issue. Plugged my keyboard(s) into the iMac (A) and it's as if I plugged "nothing" into the USB port, i.e.: the mac does not see it (even checking the USB port status using "About This Mac"/system info)... Strange, as anything else plugged into the same ports does appear, as it should.
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Matteo HermaniAug 29 '13 at 21:09